I've probably "lost" money on most of the guns I've sold.
I dont have the space or inclination to have a big collection, so when I run out of room, want something new, or find I'm not shooting something regularly, I generally sell something. Since most of the guns I've bought I've bought new, they're not worth as much when I sell them.
However, I doubt I've ever "lost" as much money selling a gun as I've "lost" feeding it while it's been in my possession.
I live in a state that allows private sales, and since I'm picky about who I sell to, and don't like having people call about adds for weeks on end I usually price them so they move.
Money comes, money goes, and I don't think my overall financial situation is all that much worse for loosing a couple hundred bucks every year or so.